3-Tiers of Client Server¶
Steven F. Lott¶
Consultant, CTG¶
1. What’s your worst nightmare?¶
Large, Complex Systems¶
Large Database (over 2 Gb)
Complex Graphical User Interface (GUI)
Large user base
- 100’s, 1000’s, Growing daily
- Untrained users, casual users, infrequent users
High availability
- 7x24
Network
- Wide-Area (WAN)
What can you do?¶
- Panic
- Flee
- Subcontract
- Use Single Vendor Solutions
- Microsoft, Novell, Oracle, IBM, ?
- Switch Careers
- Separate the Concerns
That’s Okay, but...¶
Separating Concerns allows us to
- Focus on issues
- Use standard interfaces between components
- Use specialists’ skills
How do you put it all together?¶
- Why can’t there be an off-the-shelf solution?
- Why such an integration headache?
- What happened to mainframes?
- How will it all work?
“Reference Architecture”¶
What is an architecture?
- List of components
- Hardware
- Software
- Connectivity
Specifications
- Performance, size, interface compatibility
Version Identification
Part of Design, Build and Test Efforts
Where do I get reference architectures?¶
CS/10000
- A software product that defines many combinations
Trade Magazines
Big DB Box Example¶
- Big UNIX Engine (RS/6000, Alpha, SPARCserver, etc.)
- 128-256MB RAM,
- 8Gb Disk (8 1G drives)
- ORACLE 7.2.2 (or Informix or Ingres)
- Intersolve’s ODBC
- x MIPS, y TPS
2. Typical Architectural Patterns¶
Some Patterns¶
- Server Only
- Workstation Only
- 2-Tier Client-Server
- Fat Client
- Thin Client
- 3-Tier Client-Server
Server Centric¶
Mainframe & Terminals
Issues
- The old “single-point of failure” mode
- Maintenance downtime limits availability
- Access can be limited by network bandwidth
- GUI + DB too much work for any processor
Easy to administer
- 1 app. and 1 DB in 1 location
- Handles big DB’s
Workstation Only¶
PC with MS-Access or FoxPro
Issues
- No central DB
- No security
- Configuration management nightmare
- How do you get consistency in uncontrollable desktops?
- 100’s or 1000’s of users?
Easy and Fun development environment!
Client-Server¶
Generally, workstation plus server
Simple as a file server with a central “DB” file
Issues
- Processing – Server side or client side?
- Security – Who can do what to the server?
- Configuration Control – who is running what?
Two variants - fat client vs. thin client
Fat Client¶
Everything done on the workstation
- Visual Basic, Power Builder, Delphi
- Server has a skinny database
Issues
- Client growth impossible
- Network traffic server to client
- Long-running (background) processing
- Complex, fragile applications
- Shared development on PC’s?
- Runtimes on PC’s
- Software distribution and configuration control
Thin Client¶
Everything done on the server
- Server has all application programs and database
- Client has presentation only
X-windows, Web Browsers, ActiveX controls for VB
- Reduces the server’s load associated with a GUI
Issues
- Server load, server growth
- Complex fragile application
3-Tiers¶
Focus the processor loads
- Client side does GUI and only GUI
- App. Server does application processing
- DB Server does DB and only DB
Example 1¶
- PC running Oracle Forms 4.5
- SQL*Net over TCP/IP to Oracle7 RDBMS running PL/SQL stored procedures
- SQL*Net over TCP/IP to Oracle7 RDBMS with actual data objects
Example 2¶
- PC running NetScape 3.0 with Java Applets
- TCP/IP to Server running Java Applications
- JDBC to Oracle7 RDBMS
Example 3¶
- Visual C++ running on PC
- CORBA over TCP/IP to application server running C++ applications
- CORBA over TCP/IP to OODB (Ontos or O2) with persistent objects
Example 4¶
- VB on PC
- TCP/IP to application server with C++ applications
- ODBC over TCP/IP to RDBMS with actual database
Why are 3 tiers so good?¶
Client is small, easy to configure, support & replace
- Simple solution deployed everywhere
- Minimal support
Appl. server is Centrally administered
- No configuration nightmare
- Plenty of room for growth
- Background and batch processing is trivial
- Security via Unix (Or NT or whatever)
DB server highly optimized
- Expandable “disk farm”
- No other processing on this box
3. Tech-Talk¶
How they Connect¶
VB client to C++ app. server
C++ app. server to Oracle RDBMS
Issues
- VB not object-oriented
- RDBMS not object-oriented
Benefits
- Huge Speed
- Thin Client
VB to C++ app. Server¶
Invent a new TCP/IP protocol for your application
- Define all the transactions and responses
- You have to do it anyway as part of analysis
Get Comer’s book and write a TCP/IP server
- Only a hundred lines of simple C
Write your C++ application server
Write VB client presentation
VB Client Focus¶
VB does ONLY presentation
- No business processing
- No direct access to RDBMS
User actions become transactions
- send through TCP/IP to app. server
- Responses are displayed
C++ App. Server Focus¶
C++ does all business processing
- Must handle all requests from VB and provide a response
- Keeps the central transaction journal
- And debugging log
Must be very expandable
- Should have a unique class for each type of transaction
C++ to RDBMS¶
- ODBC or Oracle’s Pro*C 2.0
- Create a “mixin” or strategy design pattern to implement SQL access for C++ classes
- Transactions will create objects from fetches of table rows
- Changes to objects will lead to updates and inserts
- Destruction of an object will lead to a database delete
- Much processing should be PL/SQL in the DBMS
Caution¶
- Be careful of keeping C++ objects which are supposed to reflect DB actions
- DB can’t notify C++ when underlying DB object has changed
- C++ must create objects based on DB fetches, do work, update DB with final object state
Also¶
Objects have unique identity
- Irrespective of any Relational Key values
You may need to introduce surrogate keys
- Meaningless identifiers for each row
- Implements a formal “Object ID”
- Use Oracle Sequence Generators to guarantee uniqueness
4. How do I do it?¶
Learning¶
Learn a little about Object Oriented Analysis and Design
- DB Server has “Objects” and their static attributes
- App. Server has access methods for the objects
- Client has presentation which displays objects and activates processing methods.
Learn about Dynasty and Forte
- tools to build large 3-tier systems
Details depend on your role
I’m a Consultant¶
I help with package selection, vendor selection, etc.
Be sure you can separate buzz-word compliance from a real architecture
- Where is security handled, which tier?
- How can high-availability be achieved?
- Where is their architecture scaleable?
- Can they add more app servers?
- Can they add more DBMS servers?
Be sure each component plays to its strengths
Determine the level of standards compliance
I’m an Integrator (Installer)¶
I install hardware and software
Be sure you know how many boxes are appropriate for what types of loads
- How servers (app. & DB), what size?
- What is the transport protocol?
- How do I test connectivity to be sure everybody can talk?
- How do I assure service levels? What kinds of spares do I need?
- Backups? How often?
- Recovery? How Difficult?
I’m a Developer (Designer, Architect)¶
I create new software or modify software
Be sure you know how responsibilities are allocated
- Does the transport protocol timeout, hang or crash when a host dies?
- Is there a transaction manager to handle load leveling?
- Does the DBMS do locking automatically?
- Can SQL clone a table or do I have to write a cursor loop to select and insert each row?
- Where in the architecture feature <I>X </I>is handled?
Final Notes¶
The World-Wide Web¶
For an outstanding 3-tier architecture, log onto the web. Any site with a search engine has three tiers.
- Your PC is presentation
- The Web server (www.altavista.com) does the application
- Another box you can’t see has the database
http://www.altavista.digital.com/
Oracle’s WebServer 2.0 is an convenient integration of Web server and DBMS connectivity
But I don’t want to emulate the Web¶
Okay, build your client in VB, PB, Visual Objects, Visual C++, Java...
Completely isolate all non-presentation logic into application server libraries
- This is really, really hard–it takes a lot of discipline
- Developers always want an easy way out and will circumvent this to meet their deadlines
Completely isolate all data storage into the database
- This is easy, now that everyone accepts SQL as the one true data access method
Are there any risks?¶
Yes, you will always have trouble with creating a clean application server
- Any application services layer is better than none
- Build the GUI last, after you understand the application
Your products may not cooperate if they are non-standard
Only pure TCP/IP will really plug and play
- IPX/SPX is high risk
Microsoft created most of the problem with desktop compatibility
- Trusting them to provide a solution doesn’t seem right
What will the rewards be?¶
CSX rolls out a major system in Java for under $1M
- Upgrading their old DOS system would have been $7M
One of my clients had a PB job that would have taken a stand-alone P90 3 months to complete
- Using Windows NT server, WFW client and SQL changes we got it down to 3 hours
6 Key Points¶
- Big, growing applications require scalable architecture
- Separate 3 tiers: client, application server, database server
- Client does only presentation
- TCP/IP Protocol to application server
- Application Server does all processing
- ODBC (or SQL*Net) protocol to database server
- Know Standards Compliance and Performance
- Learn from WWW applications (CGI’s, Java, etc.)
What Next?¶
- Java (http://java.sun.com)
- Javasoft (http://www.javasoft.com)
- Platinum Technologies (http://www.platinum.com)
- Novell’s Vision of Client-Network Computing for Developers (http://www.novell.com/java/paper2.htm)
- Symantec’s Café development tools (http://cafe.symantec.com)
- Microsoft ActiveX components (http://www.microsoft.com/devonly/community/cbisv_1.htm)
- Dynasty’s 3-tier software engineering product (http://www.dynasty.com/product)
- Forté’s 3-tier product (http://www.forte.com/forte)
- Rational’s Object Technology: OMT, Booch and Use-Case tools (http://www.rational.com/ot)
